Need help fast on hs geometry please

[tex]\begin{gathered}15 = 4x + 3 \\ = > 4x = 15 - 3 \\ = > 4x = 12 \\ = > x = 3\end{gathered}
15=4x+3
=>4x=15−3
=>4x=12
=>x=3[/tex]
Answers:
3, 15
Hope you could get an idea from here.
Doubt clarification - use comment section.